Hi Im David, I'll do my best to help you with your issue, it sounds to me like the passenger temp actuator may be searching. this happens with age and operation. the arms that control these actuator are made of plastic and sometimes break or hang up. they also wallow out inside the actuator making it slip.

they are actually called blend door actuators. there are so many different synonyms for these actuators anymore, it is even confusing for us. best way to tell is by ear. when this does happen, do you hear a clicking sounds behind the dash?